Filtration is a widely used process whereby a slurry or solid liquid mixture is forced through a media, with the solids retained on the media and the liquid phase passing through. This process is generally well understood in the industry. Examples of filtration types include depth filtration, pressure and vacuum filtration, and gravity and centrifugal filtration.
Both pressure and vacuum filters are used in the dewatering of mineral concentrates. The principal difference between pressure and vacuum filters is the way the driving force for filtration is generated. In pressure filtration, overpressure within the filtration chamber is generated with the help of e.g. a diaphragm, a piston, or external devices, e.g. a feed pump. Consequently, solids are deposited onto the filter medium and filtrate flows through into the filtrate channels. Pressure filters often operate in batch mode because continuous cake discharge is more difficult to achieve.
The cake formation in vacuum filtration is based on generating suction within the filtrate channels. The most commonly used filter media for vacuum filters are filter cloths and coated media, e.g. the ceramic filter medium. Although several types of vacuum filters, ranging from belt filters to drums, exist, only the specifics of rotary vacuum disc filters are included here.
Rotary vacuum disc filters are used for the filtration of relatively free filtering suspensions on a large scale, such as the dewatering of mineral concentrates. The dewatering of mineral concentrates requires large capacity in addition to producing a cake with low moisture content. Such large processes are commonly energy intensive and means to lower the specific energy consumption are needed. The vacuum disc filter may comprise a plurality of filter discs arranged in line co-axially d around a central pipe or shaft. Each filter disc may be formed of a number of individual filter sectors, called filter plates, that are mounted circumferentially in a radial plane around the central pipe or shaft to form the filter disc, and as the shaft is fitted so as to revolve, each filter plate or sector is, in its turn, displaced into a slurry basin and further, as the shaft of rotation revolves, rises out of the basin. When the filter medium is submerged in the slurry basin where, under the influence of the vacuum, the cake forms onto the medium. Once the filter sector or plate comes out of the basin, the pores are emptied as the cake is deliquored for a predetermined time which is essentially limited by the rotation speed of the disc. The cake can be discharged by a back-pulse of air or by scraping, after which the cycle begins again. Whereas the use of a cloth filter medium requires heavy duty vacuum pumps, due to vacuum losses through the cloth during cake deliquoring, the ceramic filter medium, when wetted, does not allow air to pass through which does not allow air to pass through, which further decreases the necessary vacuum level, enables the use of smaller vacuum pumps and, consequently, yields significant energy savings.
The filter plate is affected by slurry particles and extraneous compounds, especially in the field of dewatering of mineral concentrates, and as the replacement of a plate can be expensive, the regeneration of the filter medium becomes a critical factor when the time-in-operation of an individual filter plate needs to be increased. The filter medium is periodically regenerated with the use of one or more of three different methods, for example: (1) backwashing, (2) ultrasonic cleaning, and (3) acid washing. Whereas the regenerative effect of backwashing and ultrasound are more or less mechanical, regeneration with acids is based on chemistry. As another benefit of a ceramic filter medium, the ceramic filter plate is mechanically and chemically more durable than, for example, filter cloths and can, thus, withstand harsh operating conditions and possible regeneration better than other types of filter media. These attributes allow for chemical regeneration of the filter plates with acids, whereas a cloth would have to be discarded, after being blinded by particles, and replaced several times during a year's operation.
Filter plate breakages happen once a while in operation when the filter plates are old, something gets stuck to scrapers or for some other reason. There is a significant risk that breakage of one plate causes domino effect as the pieces of the broken plate may break neighbouring filter plates etc, which may lead to a massive “plate salad” of broken plates, and the filter is out of operation for many days. In the worst case the “plate salad” might cost hundreds of thousands of dollars for the user due to cost of the filter plates and the shutdown time of the process.